You’ve likely run into a BCC limit issue which is pretty common. First, make sure you can send to one user successfully. Once you can do that, send another email to 2-3 users as there is a different path through the code for one user than there is for more than one user.

If both those tests work, check with your hosting provider and see if there are any limitations on sending mail from your web server. Some things to check for a max emails per hour or max recipients per email, things like that.

Yes, unfortunately it actually looks like cPanel has limited my emails. The first app I tried broke my cPanel emailing and I now have 10,000+ emails in queue. Ended up purchasing GoDaddy’s email marketing app.

In my experience hosting providers impose two types of limits, there may be others but these are the two I’ve encountered most frequently helping people deploy Email Users.

Limitation on the email recipient addresses any single email can be sent to, particularly with respect to the BCC header.

Limititation on the number of email messages which can be sent in some period of time, usually an hour (limited to 5-10 email messages per hour or something similar).

Email Users has a setting which can help with the first limitation – the BCC Limit setting. It limits the number of recipient addresses which appear in any one email. If you are sending an email to 537 recipients and the BCC Limit is set to 100, Email Users will send out six (6) email messages. Message #1 will go to recipients 1-100, #2 to recipients 101-200, #3 to recipients 201-300, #4 to recipients 301-400, #5 to recipients 401-500 and #6 to recipients 501-537.

This solution works well except in the case where the second limitation kicks in – a limit on the number of email messages which can be sent in a period of time. Email Users does not contain any scheduling or queuing system to spread the sending of emails out over a period of time.

When your hosting provider imposes this sort of limitation, you’ll need to find another solution to help. I’ve heard reports from a few users that deploying an SMTP plugin which funnels mail through GMail or another authenticated email account can work around this limitation but I don’t have any personal experience doing so.

Hope this helps clarify what is and isn’t possible to work within the confines of hosting provider limitations using Email Users.